A comprehensive examination is proposed of the effects of age, physical capabilities, functional status, neuropsychological factors and Parkinson's Disease (PD) on the biomechanics of mobility task performances. We will quantify physical component and neuropsychological component test scores with performances in a series of 26 mobility tasks that probe abilities to ambulate, rise from a chair and maintain upright posture in the presence of perturbations. We will study in detail the biomechanical requirements of performing those 26 and selected other tasks. We will determine relationships among the component test score, performance profile and biomechanical requirement data, and test a number of hypotheses concerning those relationships so as to achieve major insight into fundamental aspects of mobility in old adults. Understanding of how functional mobility is affected by aging, physical capabilities, neuropsychological status and task performance biomechanical requirements will enable earlier detection and more precise assessment of declines in performance capabilities. It will show what aspects of task performances are truly critical to performance abilities and thus lead to improved intervention. We will measure physical and neuropsychological component scores and task performance abilities among ten groups of subjects with various ages (55 to 84 yr) and functional status (functionally able to moderately disabled). Three of these will be PD groups. Task performance biomechanical requirements will be studied in three groups formed from among the original ten. Detailed biomechanical analyses of other task performances will be made in three other subject groups. There will be four Projects: (1) Simple Assessment of Mobility in Old Adults, (2) Biomechanics of Ambulation in Old Adults, (3) Biomechanics of Chair Rise in Old Adults, and (4) Biomechanics of Postural Maintenance in Old Adults. Each of these projects will be supported by three Cores: (A) Administrative, (B) Human Subjects/Data Analysis and (C) Neuropsychology. The team of investigators will include personnel with expertise in Biomechanics, Biostatistics, Geriatric Medicine, Neurology, Neuropsychology, Otolaryngology, Physical Therapy and Vision.